This invention concerns adaptive channel equalization of an image representative signal which may be processed by an image processing and display device such as a television receiver.
The recovery of data from modulated signals conveying digital information in symbol form usually requires three functions at a receiver: timing recovery for symbol synchronization, carrier recovery (frequency demodulation to baseband), and channel equalization. Timing recovery is a process by which a receiver clock (timebase) is synchronized to a transmitter clock. This permits the received signal to be sampled at the optimum point in time to reduce the chance of a slicing error associated with decision-directed processing of received symbol values. Carrier recovery is a process by which a received RF signal, after being frequency downconverted to a lower intermediate frequency passband (e.g., near baseband), is frequency shifted to baseband to permit recovery of the modulating baseband information.
Many digital data communications systems employ adaptive equalization to compensate for the distortion effects of changing channel conditions and disturbances on the signal transmission channel. The equalization process estimates the transfer function of the transmission channel and applies the inverse of the transfer function to the received signal so as to reduce or eliminate the distortion effects. Channel equalization typically employs filters that remove amplitude and phase distortions resulting from a frequency dependent time variant response of the transmission channel, for example, to thereby provide improved symbol decision capability. Equalization removes baseband intersymbol interference (ISI) caused by transmission channel disturbances including the low pass filtering effect of the transmission channel. ISI causes the value of a given symbol to be distorted by the values of preceding and following symbols, and essentially represents symbol xe2x80x9cghostsxe2x80x9d since ISI includes advanced and delayed symbols with respect to a reference symbol location in a given decision region.
An adaptive equalizer is essentially a digital filter with an adaptive response to compensate for channel distortions. Several well-known algorithms are available for adapting the filter coefficients and thereby the filter response to converge the equalizer. In the process of converging the equalizer coefficients, large step sizes are typically used to allow the coefficients to converge faster during an initial adaptation stage, while small step size changes are used at other times to prevent the coefficient values from changing rapidly. The present invention is directed to a process that supports the efficient use of such step size changes.
In accordance with the principles of the present invention, adaptive equalization is facilitated by dynamically changing the equalizer filter coefficient step size during the adaption process as a function of the packet error rate such as may be evidenced by the output of an FEC error detecting and correcting network.